FERTILIZERS 107 



importance to investigate and extend our knowldege of the effect 

 of soil amendments upon the many factors in crop production. 

 With a knowledge of the effect of fertilizers upon the physical, 

 chemical and biological factors in crop production, and of the 

 nature of the interdependence of these factors, will come the 

 ability to manage intelligently the individual field for the part- 

 icular crop. This knowledge can only come by attacking the 

 problem from the dynamic view-point, and so far as the soil 

 factors are concerned, they can apparently be studied best as 

 they affect the properties of the soil solution. 



While it seems certain that some fertilizer effects are directly 

 upon the soil and secondarily upon the plants, it cannot be doubted 

 that in others, the phenomena are more directly concerned with 

 the absorption by and the metabolism within the plant and until 

 these plant processes are better understood, nothing approaching 

 a satisfactory practice can be anticipated. Why and how plants 

 exercise the selective powers they appear to possess are funda- 

 mental questions yet to be answered. The important effects 

 sometimes produced by adding to the nutrient medium such 

 substances as manganese salts which are not necessary to the 

 growth of the plant, can no more be neglected than the study of 

 the phosphorus needs. The presence in the soil universally of 

 substances other than the recognized mineral nutrients, 1 may 

 very well have a significance for plant production hitherto 

 unsuspected, for the fact that an organism can continue to 

 function in the absence of a substance is no argument, much 

 less proof, that it would not function better with that substance 

 present. Recent investigations, showing that animal organisms 

 are sometimes more resistant to certain toxins and diseases under 

 starvation conditions or when ingesting substances unnecessary 

 to normal development, suggest the possibility at least of similar 

 phenomena with plants. It is at any rate clear that the practical 

 problem of the best production of plants from soils is not merely 

 one of providing a relatively large supply of potassium, phos- 

 phorus and nitrogen. 



1 See, for instance, Barium in soils, by G. H. Failyer, Bull. No. 71, 

 Bureau of Soils, U. S. Dept. of Agriculture, 1910. 



